The present invention relates generally to internal combustion engines, and, in particular, to cylinder crankcase assemblies for four-stroke engines.
Previously, small hand-held lawn and garden implements, chainsaws, and small vehicles were often powered using two-stroke engine technology. However, due to increasingly stringent environmental emission controls, the use of four-stroke engines in these appliances and vehicles has become more common. Unlike two-stroke engines, four-stroke engines do not supply fresh fuel to the combustion chamber while also scavenging the combustion products from the previous stroke. Therefore, four-stroke engines have lower hydrocarbon emissions.
Both two and four-stroke engines typically consist of a crankcase, cylinder block, and cylinder head. Generally, the crankcase, cylinder block, and cylinder head need to be joined together using mechanical fasteners, thereby necessitating both additional fasteners and precisely machined fastener holes. Engines composed of separate cylinder blocks, cylinder heads, and crankcases also require sealing gaskets. These additional components add extra weight to the engines and also present greater potential for gasket failures.
To improve engine emissions while avoiding the short-comings of engines made from separate components, it may be desirable to produce a monolithic four-stroke crankcase, cylinder block, and cylinder head. However, because four-stroke engines require an additional valve-train and valve mechanism, casting such monolithic engines is more difficult than the corresponding two-stroke engines. To overcome these challenges, monolithic cylinder blocks and crankcases have been designed having half-crank crankshafts with L-head (flat-head) valve trains, or full crankshafts with wet-type or dry-type belt driven overhead valves. As is known in the art, L-head valve arrangements provide poor fuel economy, and full crankshafts increase the weight of the engine.